Process for producing preform for glass fiber and process for producing glass fiber

ABSTRACT

A process for producing a preform for a chalcogenide glass fiber which comprises inserting a cladding tube having contained therein a chalcogenide glass rod for core into a quartz tube having at its bottom a nozzle having an aperture smaller than the outer diameter of the cladding tube, locally heating the bottom of the quartz tube and pulling the cladding tube having contained the glass rod for core and a process for producing a chalcogenide glass fiber by heating and drawing the preform thus obtained, by which processes the devitrification of glass and the generation of bubbles in the core glass or at the core glass-cladding glass interface can be prevented and the adhesion between the core glass and the cladding glass can be improved. In particular, when the glass material for core does not contain Ge, a chalcogenide glass fiber having such a core-cladding structure that the transmission loss of the glass fiber when infrared light pass through the fiber is small and the mechanical strength is high.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing a preform for achalcogenide glass fiber of a core-cladding structure, a process forproducing a chalcogenide glass fiber suitable to the transmission ofinfrared rays, particularly rays of 1 to 14 μm by using the abovepreform, and a chalcogenide glass fiber of a core-cladding structureexcellent in infrared ray-transmissibility, particularly suitable totransmission of infrared signal light.

2. Prior Art

A cast method has been known as a method for producing a preform of acore-cladding unitary structure using chalcogenide glass. This castmethod comprises casting molten core glass into a tube of claddingglass.

As other methods, JP-A-1-230,440 proposes a method for producing apreform of a core-cladding unitary structure by inserting a claddingglass tube into a quartz tube closed at its bottom, further inserting acore glass rod into the cladding glass tube, and heating the outer sideof the quartz tube of the resulting assembly in this state by means of aring heater while reducing the pressure in the space between the coreglass rod and the cladding glass tube and applying a pressure to thespace between the cladding glass tube and the quartz tube, therebyuniting the core glass and the cladding glass.

Chalcogenide glass fibers can be obtained by heat and drawing thepreform obtained by the above cast method or the method ofJP-A-1-230,440.

Moreover, there has been also known a method for producing achalcogenide glass fiber directly from a core glass rod and a claddingglass tube without preparing a preform. As this method, JP-A-1-226,748proposes a method for producing a glass fiber by inserting a claddingglass tube having contained therein a core glass rod, into a quartz tubehaving at its bottom a nozzle having an aperture smaller than the outerdiameter of the cladding glass tube and drawing the resulting assemblywhile locally heating the lower portion of the assembly than the lowerend of the quartz tube while controlling the gas pressure in the spacebetween the cladding glass tube and the quartz tube so as to becomehigher than the gas pressure in the space between the core glass rod andthe cladding glass tube.

It is well known that chalcogenide glass is thermally unstable.Accordingly, in the method for producing a chalcogenide glass fiber bythe above-mentioned cast method, there is such a problem that the glassis devitrified in the course of the production of a preform. Inaddition, the molten core glass easily takes in bubbles during thecasting and these bubbles remain as broths at the interface of the coreglass and the cladding glass. When the preform containing these brothsis formed into fibers, the broths have become a cause of increasing thetransmission loss. By this cast method, it has been impossible toproduce a preform for a single mode fiber in which the core diameter ismuch smaller than the cladding diameter.

The method stated in JP-A-1-230,440 for obtaining a preform by unitingthe core glass and the cladding glass under control of the pressure in aquartz tube whose bottom is closed can considerably inhibit thedevitrification of glass and the generation of bubbles in the core glassor at the core glass-cladding glass interface as compared with the castmethod. However, the preform obtained cannot be said to be sufficient inadhesion between the core glass and the cladding glass, and animprovement in adhesion has been desired. Also, this method forproducing a preform was unable to produce a preform for a single modefiber in which the core diameter is much smaller than the claddingdiameter.

On the other hand, in the method stated in JP-A-1-226,748 for producinga glass fiber directly from a core glass rod and a cladding glass tubewithout preparing any preform, there has been such a problem that sincethe cladding glass tube having contained therein the core glass rod mustbe heated at a high drawing temperature from the beginning, thechalcogenide glass tends to be devitrified and the composition tends tobe changed by volatilization. The said method for the direct productionof a fiber has such a disadvantage that it is difficult to obtain asingle mode fiber in which the core diameter is much smaller than thecladding diameter. Moreover, according to the method for the directproduction of a fiber, when it is intended to produce fibers havingdifferent diameters, a plurality of quartz tubes having drawing nozzleshaving correspondingly different diameters must be used and when a fiberhaving a specific diameter has become necessary, it is difficult toobtain such a fiber in a short period of time.

Furthermore, as to the chalcogenide glass fiber of a core-claddingstructure, the technique stated in, for example, JP-A-3-8,742 hasheretofore been known. This technique intends to provide a powertransmission fiber in which each of the core glass and the claddingglass is composed of three elements of As (arsenic), S (sulfur) and Ge(germanium) for enhancing particularly the heat resistance and Se(selenium) is substituted for a part of the S (sulfur) in the core glassfor controlling the numerical aperture (NA) of the fiber.

However, in the case of the above technique, in order to enhance theheat resistance of a power transmission infrared fiber, Ge element iscontained in both the core glass and the cladding glass, and hence, thefact that both contain Ge makes it basically difficult to make thetransmission loss lower than a certain level. Also, since Ge iscontained in both the core glass and the cladding glass, a sufficientdifference in thermal expansion between the two is not obtained in theformation of a fiber and hence it is impossible to enhance themechanical strength of the fiber.

In addition, in the case of the above core-cladding structure, there issuch a problem that infrared rays passing through the cladding sidebecome a noise when an optical signal is transmitted and the opticalsignal to be transmitted in the core undergoes a disturbance.

SUMMARY OF THE INVENTION

The first object of this invention is to provide a process for producinga preform for a chalcogenide glass fiber by which the problems admittedin the above-mentioned prior art methods including devitrification ofglass, generation of bubbles in the core glass or at the coreglass-cladding glass interface, incomplete adhesion at the coreglass-cladding glass interface and the like have been solved.

The second object of this invention is to provide a process forproducing a preform for a chalcogenide glass fiber suitable forobtaining a single mode fiber in which the core diameter is much smallerthan the cladding diameter.

The third object of this invention is to provide a process for producinga chalcogenide glass fiber which makes it possible to obtain, from thepreform obtained by the processes for producing a preform mentioned inthe above first and second objects, a fiber whose diameter is varieddepending upon the drawing conditions in a short period of time withoutusing the quartz tube having spinning nozzles different in diameter andwithout causing devitrification and composition change due tovolatilization.

The fourth object of this invention is to provide a chalcogenide glassfiber having a core-cladding structure by which the transmission loss offiber during the transmission of an infrared signal light has beenminimized and the mechanical strength of fiber has been enhanced.

The fifth object of this invention is to provide a chalcogenide glassfiber having a core-cladding-cladding structure which has been protectedfrom disturbing light to improve transmission characteristics.

According to this invention, there is provided a process for producing apreform for a chalcogenide glass fiber which comprises inserting acladding chalcogenide glass tube having contained therein a chalcogenideglass material for core, into a quartz tube having at its bottom anozzle having an aperture smaller than the outer diameter of thecladding glass tube, locally heating the bottom of the quartz tube andpulling out the cladding glass tube having contained therein the glassmaterial for core this process is referred to hereinafter as theproduction process (1) in some cases!.

According to this invention, there is further provided another processfor producing a preform for a chalcogenide glass fiber which comprisesinserting, as a glass material for core, the preform obtained by theabove production process (1) into a cladding glass tube, repeating atleast one time the production process (1) mentioned above using theresulting assembly to obtain a preform for a single mode fiber in whichthe core diameter is extremely small as compared with the claddingdiameter this process is referred to hereinafter as the productionprocess (2) in some cases!.

According to this invention, there is still further provided a processfor producing a chalcogenide glass fiber which comprises polymer-coatingthe preform obtained by the production process (1) or (2) with anpolymer-coating material and heat and drawing the coated preform.

This invention further provides a chalcogenide glass fiber having acore-cladding structure in which the core glass is composed of twoelements of As (arsenic) and S (sulfur) and the cladding glass (referredto hereinafter as the first cladding glass in some cases) is composed ofthree elements of As (arsenic), S (sulfur) and Ge (germanium) and also achalcogenide glass fiber having a core-cladding-cladding structure inwhich the above fiber having a core-cladding structure is covered with asecond cladding glass having a refractive index lower than that of theabove core glass but higher than that of the above first cladding glass,the second cladding glass being composed of three elements of As(arsenic), S (sulfur) and Ge (germanium).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a crosssectional view showing the process for producing apreform of this invention.

FIG. 2 is a crosssectional view showing the process for producing afiber of this invention.

FIG. 3 is a crosssectional view showing the preform obtained in Example4.

FIG. 4 is a crosssectional view showing the fiber obtained in Example 4.

FIG. 5 is an outline cross section of an apparatus for producing thechalcogenide glass fiber of this invention.

FIG. 6 is a crosssectional view of the fiber obtained in Example 5.

FIG. 7 is a crosssectional view of the fiber obtained in Example 6.

In FIGS. 1 to 4, 1 refers to a quartz tube, 2 to a core rod, 3 to acladding glass tube, 4 to the bottom of the quartz tube, 5 to a nozzle,6 to the upper part of the quartz tube, 7 to a pressurizing tube, 8 tothe space between the quartz tube and the cladding tube, 9 to the spacebetween the cladding glass tube and the core rod, 10 to apressure-reducing tube, 11 to a preform, 12 to a heat-shrinkable resintube, 13 to a ring heater, 14 to a fiber, 15 to the tip of the preform,16 to a weight, 17 to core glass, 18 to cladding glass of the firstcomposition, 19 to cladding glass of the second composition and 20 to aheat-shrunk resin layer.

In FIGS. 5 to 7, 101 refers to a core rod, 102 to a cladding tube, 103to a cover, 104 to a resin tube, 105 to a resin tube, 106 to a suckingresin tube, 107 to a crucible, 108 to a nozzle, 109 to an inlet forinert gas, 110 to a suction port, 111 to an inlet for inert gas, 112 toan atmosphere conditioning chamber, 113 to a fiber diameter measuringinstrument, 114 to a resin coater, 115 to a UV irradiation chamber, 116to print rollers, 117 to a local heater, 120 to core glass, 121 to firstcladding glass and 122 to second cladding glass.

DETAILED DESCRIPTION OF THE INVENTION

First of all, an explanation is made of the production process (1)below.

The production process (1) comprises inserting a cladding chalcogenideglass tube having contained therein a chalcogenide glass material forcore, into a quartz tube having at its bottom a nozzle having anaperture smaller than the outer diameter of the cladding glass tube,locally heating the bottom of the quartz tube and pulling out thecladding glass tube having contained therein the glass material forcore, thereby obtaining a preform for a chalcogenide glass fiber.

According to the production process (1), unlike the conventional castmethod in which the whole of the glass material for core is heated andmelted, the glass material is heated only at the bottom of the quartztube, whereby the problems of devitrification of glass and generation ofbubbles in the core glass or at the core glass-cladding glass interfacewhich problems are seen in the conventional cast method have beensolved.

Also, according to the production process (1), the core glass and thecladding glass are forcibly melt-united in such a dynamic state that thecladding glass tube having contained therein the glass material for coreis heated and then pulled from the nozzle at the bottom of the quartztube, thereby obtaining a preform. Therefore, as compared with themethod stated in JP-A-1-230,440 in which a cladding glass tube havingcontained therein a glass material for core is statically heated in abottom-closed quartz tube having no nozzle at the bottom to unite thecore glass and the cladding glass, the production process (1) isadvantageous in that a preform excellent in adhesion between the coreglass and the cladding glass is obtained.

In the production process (1), it is preferable that the pressure in thespace (i) between the glass material for core and the cladding glasstube is made relatively lower than the pressure in the space (ii)between the cladding glass tube and the quartz tube, whereby thedevitrification of glass and generation of bubbles in the core glass orat the core glass-cladding glass interface can be more effectivelyinhibited and the adhesion between the core glass and the cladding glassis further enhanced. In particular, it is more preferable to keep thespace (i) under reduced pressure and the space (ii) under elevatedpressure. The pressure in the space (i) between the glass material forcore and the clad glass tube is preferably 15 Pa or less, morepreferably 1.5 Pa or less. The pressure in the space (ii) between thecladding glass tube and the quartz tube is preferably 2.0×10⁴ Pa ormore, more preferably 2.0×10⁵ Pa or more.

In the production process (1), it is preferable to control the heatingtemperature at the bottom of the quartz tube to a temperature lower thanthe drawing temperature, whereby it becomes possible to obtain a preformhaving a large diameter and also prevent the devitrification of glass inthe production of the preform.

The heating temperature (Th) at the bottom of the quartz tube isparticularly preferably controlled to a temperature 40-150° C. lowerthan the drawing temperature (Tf). The reason therefor is as follows:When Th>(Tf-40) and the temperature Th is relatively high, the thermallyunstable chalcogenide glass is easy to devitrify and crystallizationtends to be caused particularly in the core glass or at the coreglass-cladding glass interface. When the preform obtained under suchconditions is drawn into a fiber, the transmission loss of the fiberobtained becomes remarkably large, and the volatilization of the glasscomponent causes a change in composition. On the other hand, whenTh<(Tf-150) and the temperature Th is relatively low, the chalcogenideglass is not sufficiently softened, and it becomes impossible to pullout the cladding glass united with the core glass from the nozzle of thequartz tube or even if the pulling is possible, a sufficient adhesionbetween the core glass and the cladding glass is not obtained, thetransmission loss becomes large and the fiber strength becomes low.

When the cladding glass tube having contained therein the glass materialfor core is pulled from the quartz tube, it is preferable to hang aweight from the tip of the cladding glass tube. When a weight is hungthe following effects can be obtained:

(a) The preform is formed into a straight rod and hence the fiber yieldis increased when this preform is drawn into a fiber.

(b) The drawing speed of the preform becomes constant and hence thediameter of preform is stabilized and the same effect as in (a)(increase of the fiber yield) is obtained.

The weight can be suitably determined and generally, a weight of 50 g ormore is preferred.

An explanation is further made of the production process (2) below.

The production process (2) uses the preform for a chalcogenide glassfiber obtained by the production process (1) as a glass material forcore and is characterized by repeating at least one time the productionprocess (1) using a cladding glass tube having the preform containedtherein, whereby a preform for a single mode fiber having a corediameter much smaller than the cladding diameter can be obtained.

The production process (2) is basically the repetition of the productionprocess (1) and hence, as a matter of course, the advantage obtained bythe production process (1) (inhibition of devitrification of glass,inhibition of generation of bubbles and improvement of adhesion) canalso be brought about by carrying out the production process (2).

The preforms for a glass fiber obtained by the production processes (1)and (2) can be drawn in a conventional manner into glass fibers, and itis possible to form a protective coating layer consisting of a resin onthe glass fiber thus obtained by passing the glass fiber through avessel containing the resin. The resin used is preferably a UV curableresin; however, this is not critical and any resin can be used as far asit has a fiber-protective function. For example, a thermosetting resincan be used.

Next, the process for producing a chalcogenide glass fiber is explained.

The process for producing a chalcogenide glass fiber comprisesadhesion-coating the preform for a chalcogenide glass fiber obtained bythe production process (1) or (2) with an adhesion-coating material andheating and drawing the coated preform to obtain a chalcogenide glassfiber.

According to the above process for producing a glass fiber, the coatingof the preform with an adhesion-coating material makes it possible toproduce fibers having varying diameters freely by varying the processingconditions including drawing temperature, drawing speed and the likewithout using an expensive quartz tube at the time of drawing. Also, bypreviously preparing the preform, the fiber can be produced whenevernecessary in a short time. According to said process for producing aglass fiber, the adhesion-coating material applied to the preform caninhibit the composition change due to volatilization of a chalcogenideglass fiber upon heating at the time of drawing can be inhibited,whereby a chalcogenide glass fiber having the desired properties can beobtained.

The above adhesion-coating material may be any material as far as it isgood in adhesion to the preform and can prevent the chalcogenide glassfiber from being volatilized upon heating at the time of drawing. Inparticular, the use of a heat-shrinkable tube renders theadhesion-coatability on the preform particularly good and makes itpossible to markedly inhibit the glass component from being volatilizedfrom the preform. The heat-shrinkable tube is generally smoother at itsopening portion than the quartz tube and hence when the other area ofthe preform than its end portion is adhesion-coated with theheat-shrinkable tube and the preform is heated and drawn, the fiber canbe prevented from being wounded.

The heat-shrinkable tube includes (a) heat-shrinkable tubes offluoroplastics such as Teflon FEP(tetrafluoroethylene-hexafluoropropylene copolymer), polyvinylidenefluoride, Teflon PFA (tetrafluoroethylene-perfluoroalkoxyethylenecopolymers), Teflon TFE (polytetrafluoroethylene resin) and the like;(b) heat-shrinkable tubes of silicone rubbers; (c) heat-shrinkable tubesof synthetic rubbers such as neoprene, Viton and the like; (d)heat-shrinkable tubes of polyolefins such as cross-linked polyolefinsand the like; (e) heat-shrinkable tubes of vinyl chloride resins and thelike. The use of a heat-shrinkable tube having a melting point orsoftening point close to that of chalcogenide glass is preferablebecause said use makes it possible to carry out simultaneously theadhesion-coating of the preform with a heat-shrinkable tube and thedrawing of the preform into a fiber. As such a heat-shrinkable tube,there is mentioned a tube made of a fluoroplastic such as Teflon or thelike. This fluoroplastic is preferred because it is excellent inheat-conductivity, and hence, the time required for heating can beshortened and the yield can be increased as compared with the case wherea quartz tube is used.

The whole area of the preform may be coated with the adhesion-coatingmaterial, or the other area of the preform than its end portion to besubjected to heating and drawing may be coated with the polymer-coatingmaterial. In the former case, the preform and the adhesion-coatingmaterial are simultaneously subjected to heating and drawing, therebyobtaining a glass fiber having a triple layer structure of core glass,cladding glass and polymer-coating material. In the latter case, onlythe preform is subjected to heating and drawing, so that a glass fiberhaving a double layer structure of core glass and cladding glass isobtained. In this case, the double layer structure fiber after thedrawing can be passed through a vessel containing a resin to form aprotective coating layer consisting of the resin. The resin used ispreferably a UV curable resin as already mentioned above; however, anyresin such as a thermosetting resin or the like can be used as far as ithas a fiber-protecting function.

Explanations have been made above concerning the preform productionprocesses (1) and (2) as well as the fiber production process. The term"chalcogenide glass" used herein should be interpreted broadly andrefers generically to a glass comprising basically a chalcogen elementS, Se or Te among the elements of the VIB Group of the Periodic Tableand other component As, P, Sb, Si, Ge, Sn or the like. In addition, theprocess of this invention can also be applied to a glass having avolatility such as a fluoride glass.

Next, the glass fibers according to the invention are described.

In the chalcogenide glass fiber having a core-cladding structure of thisinvention, the core glass is composed of two elements of AS (arsenic)and S (sulfur) and the cladding glass is composed of the above twoelements and Ge (germanium) whereby the refractive index of the cladglass can be made lower than that of the core glass. In addition, sincethe core glass does not contain Ge, the transmission loss of the coreglass can be basically made smaller.

The thermal expansion coefficient of the cladding glass can be madesmaller than that of the core glass by incorporating Ge (germanium) intothe cladding glass, and this has such an effect that a compressionstress can be imparted to the cladding glass to heighten the mechanicalstrength. In addition, when the compositions of the core glass and thecladding glass fall outside the predetermined ranges, the glass is aptto become unstable. That is to say, when the proportion of Ge is lessthan 0.5 atm %, it is difficult to allow the cladding glass to have asufficient refractive index difference, and when the proportion of Ge ismore than 7 atm %, the cladding glass have too different properties fromthose of the core glass to be used as a preferable cladding glass.

The composition of the core glass used in the core-cladding structurechalcogenide glass fiber of this invention is preferably such that theproportion of As is 25 to 55 atm % and the proportion of S is 45 to 75atm %, the total of the composition being 100 atm %, and the compositionof the cladding glass used is preferably such that the proportion of Asis 18 to 54 atm %, the proportion of S is 46 to 80 atm % and theproportion of Ge is 0.5 to 7 atm %, the total of the composition being100 atm %.

In the chalcogenide glass fiber having a core-cladding-claddingstructure of this invention, the core-cladding structure chalcogenideglass fiber is further covered with a second cladding glass having arefractive index lower than that of the core glass but higher than thatof the first cladding glass (namely, the cladding glass of the abovecore-cladding structure) of the fiber, whereby the light having passedthrough the first (inner) clad glass is absorbed by the second claddingglass and dispersed and hence such an adverse effect can be preventedthat a disturbance is given as a noise light to the signal light havingpassed through the core. In this case, when the composition of thesecond cladding glass is limited to the predetermined range, the sameeffect is obtained as in the case of the core-cladding structurechalcogenide glass fiber.

In the core-cladding-cladding structure chalcogenide glass fiber of thisinvention, the composition of core glass used is preferably such thatthe proportion of As is 25 to 55 atm % and the proportion of S is 45 to75 atm %, the total of the composition being 100 atm %, and each of thecompositions of the first cladding glass and the second glad glass usedis preferably such that the proportion of As is 18 to 54 atm %, theproportion of S is 46 to 80 atm % and the proportion of Ge is 0.5 to 7atm %, the total of the composition being 100 atm %.

In the core-cladding fiber and core-cladding-cladding fiber of theinvention, since Ge is incorporated into the clad glass to make therefractive index lower than that of the core glass and the core glassdoes not contain Ge, the transmission loss of the core glass can bebasically made smaller. Moreover, since Ge is incorporated into thecladding glass, the thermal expansion coefficient of the cladding glassbecomes smaller than that of the core glass, and hence, when thecladding glass containing the core glass therein is drawn into a fiber,it is possible to allow the cladding glass to generate a compressionstress. That is, the enhancement of the mechanical strength of fiberbecomes possible.

Moreover, when the fiber is covered with the second cladding glass whichhas a refractive index higher than that of the first cladding glass butlower than that of the core glass, the light having passed through thefirst cladding glass can be absorbed by the second cladding glass anddispersed to prevent a disturbance from adversely affecting the signallight passing through the core glass.

WORKING EXAMPLE Example 1

(1) Preform-Production Example

First of all, an example is explained according to FIG. 1. A claddingtube 3 having inserted thereinto a core rod 2 was sealed into a quartztube 1. At the bottom 4 of the quartz tube 1, a nozzle 5 having asmaller aperture than the outer diameter of the cladding tube 3 wasprovided. In the upper part 6 of the quartz tube 1, a pressurizing tube7 connected to the space 8 between the quartz tube 1 and the claddingtube 3 was installed. A very slight amount of an Ar gas was introducedthrough the pressurizing tube 7 to fill the space 8 with the Ar gas. Onthe other hand, the pressure of the space 9 between the core rod 2 andthe cladding tube 3 was reduced to 1.5 Pa by a vacuum pump through apressure-reducing tube 10 installed so as to be connected to the space9.

The glass composition of the core rod 2 was adjusted to Ge 5 atm %/As 42atm %/S 53 atm % and the glass composition of the cladding tube 3 wasadjusted to Ge 5 atm %/As 40 atm %/S 55 atm %. In this case, the outerdiameter of the cladding tube 3 was adjusted to 12.5 mm and the innerdiameter thereof was adjusted to 10.5 mm. The outer diameter of the corerod 2 was adjusted to 9.5 mm and the aperture of the nozzle 5 of thequartz tube 1 was adjusted to 9 mm.

The vicinity of the nozzle 5 of the quartz tube 1 was heated by a ringheater 13 to 300° C. and when the tip of the cladding tube 3 wassoftened and closely contacted with the nozzle 5, an Ar gas wasintroduced through the pressurizing tube 7 to adjust the pressure of thespace 8 to 2×10⁵ Pa. When the tip 15 of the preform was extruded throughthe nozzle 5 of the quartz tube 1, a weight of 100 g was immediatelyhung from the tip 15 of the preform 11 in the predetermined manner. Thepreform 11 was pulled at a speed of 4 mm/min. The outer diameter of theresulting preform 11 was 6 mm. In the preform 11 obtained,devitrification of glass was not observed nor the generation of bubblesin the core glass or at the core glass-cladding glass interface wasobserved. The preform 11 was also excellent in adhesion at the coreglass-cladding glass interface.

(2) Glass Fiber-Production Example

A glass fiber was obtained using the preform obtained in (1) above.

As shown in FIG. 2, the periphery of the preform 11 obtained in (1)above was adhesion-coated with a heat-shrinkable tube 12 composed of afluoroplastic (Teflon FEP), and the lower periphery of the preform 11was heated to 390° C. by a ring heater 13 and drawn into a fiber 14.

The fiber 14 thus obtained had a triple layer structure of core glass,cladding glass and heat-shrunk resin, and the transmission loss at awavelength of 2.4 μm was 0.1 dB/m.

(3) Comparative Production Example

For comparison, the same core rod and cladding tube as in (1) above wereused to prepare a preform by the method stated in JP-A-1-230,440 inwhich a core rod-containing cladding tube was heated in a quartz tubeclosed at the bottom. Subsequently, the preform obtained was heated anddrawn in the same manner as in (2) above to obtain a glass fiber. Theglass fiber obtained was subjected to the same test as in (2) above tofind that the transmission loss at a wavelength of 2.4 μm was 0.4 dB/m.

For further comparison, the same core rod and cladding tube as in (1)above were used and the core rod-containing cladding tube was insertedinto a quartz tube having a nozzle at the bottom, after which a glassfiber was directly produced therefrom according to the method stated inJP-A-1-226,748 without preparing a preform. The transmission loss of theglass fiber thus obtained was 0.3 dB/m at a wavelength of 2.4 μm.

Example 2

(1) Preform-Production Example

The same procedure as in Example 1 (1) was repeated, except that theglass composition of the core rod 2 was adjusted to Ge 27 atm %/As 30atm %/Se 43 atm % and the glass composition of the cladding tube 3 wasadjusted to Ge 25 atm %/As 29 atm %/Se 46 atm %. In this case, the outerdiameter of the cladding tube 3 was adjusted to 12.5 mm, the innerdiameter thereof was adjusted to 10.5 mm, the outer diameter of the corerod 2 was adjusted to 9.5 mm and the aperture of the nozzle 5 of thequartz tube 1 was adjusted to 9 mm. The heating temperature was 400° C.and the pulling speed was 2 mm/min. The preform 11 obtained had an outerdiameter of 6.5 mm, and similarly to the preform obtained in Example 1,neither devitrification of glass nor generation of bubbles were observedand the adhesion between the core glass and the cladding glass wasexcellent.

(2) Glass Fiber-Production Example

The periphery of the preform 11 obtained in (1) above was coated with aheat-shrinkable tube 12 composed of a heat-shrinkable fluoroplastic(vinylidene fluoride resin) and the lower periphery of the preform 11was heated by a ring heater 13 to 500° C. and drawn into a fiber 14.

The transmission loss of the fiber 14 thus obtained was 0.4 dB/m at awavelength of 6 μm.

(3) Comparative Production Example

Using the same core rod and the same cladding tube as in (1) above, apreform was prepared by the method stated in JP-A-1-230,440. Then, thepreform was subjected to heating and drawing in the same manner as in(2) above to prepare a glass fiber. The glass fiber thus obtained wassubjected to the same measurement as in (2) above to find that thetransmission loss at a wavelength of 6 μm was 0.6 dB/m.

Separately, using a core rod and a cladding tube having the samerespective compositions as in (1) above, a glass fiber was directlyprepared by the method stated in JP-A-1-226,748 without preparing apreform. The glass fiber thus obtained was subjected to the samemeasurement as in (2) above to find that the transmission loss at awavelength of 6 μm was 0.55 dB/m.

Example 3

(1) Preform-Production Example

The same procedure as in Example 1 (1) was repeated, except that theglass composition of the core rod 2 was Ge 24 atm %/Se 21 atm %/Te 55atm % and the glass composition of the cladding tube 3 was Ge 23 atm%/Se 24 atm %/Te 53 atm %. In this case, the outer diameter of thecladding tube 3 was 12.5 mm, the inner diameter of the cladding tube 3was 10.5 mm, the outer diameter of the core rod 2 was 9.5 mm, and theaperture of the nozzle 5 of the quartz tube 1 was 9 mm. The heatingtemperature was 400° C. and the pulling speed was 2 mm/min. The outerdiameter of the preform 11 obtained was 8.0 mm and, similarly to thepreform obtained in Example 1, neither devitrification of glass norgeneration of bubbles were observed. The adhesion between the core glassand the cladding glass was excellent.

(2) Glass Fiber-Production Example

The periphery of the preform 11 obtained in (1) above was coated with aheat-shrinkable tube 12 composed of heat-shrinkable fluoroplastic(Teflon FEP), and the lower periphery of the preform 11 was heated to440° C. by a ring heater 13 and drawn into a glass fiber 14.

The transmission loss of the glass fiber thus obtained was 0.4 dB/m at awavelength of 8 μm.

(3) Comparative Production Example

Using a core rod and a cladding tube having the same respectivecompositions as in (1) above, a preform was prepared by the methodstated in JP-A-1-230,440 and then heated and drawn in the same manner asin (2) above to obtain a glass fiber. The glass fiber thus obtained wassubjected to the same measurement as in (2) above to find that thetransmission loss at a wavelength of 8 μm was 0.6 dB/m.

Separately, using a core rod and a cladding tube having the samerespective compositions as in (1) above, a glass fiber was directlyprepared without preparing a preform by the method stated inJP-A-1-226,748. The glass fiber obtained was subjected to the samemeasurement as in (2) above to find that the transmission loss at awavelength of 8 μm was 0.55 dB/m.

Example 4

(1) Preform-Production Example

The same procedure as in Example 1 (1) was repeated, except that theglass composition of the core rod 2 was As 40 atm %/S 60 atm % and theglass composition of the cladding tube 3 was Ge 2 atm %/As 37 atm %/S 61atm % (the first composition). In this case, the outer diameter andinner diameter of the cladding tube 3 were 15.5 mm and 8.0 mm,respectively, the inner diameter of the core rod 2 was 7.5 mm, and theaperture of the nozzle 5 of the quartz tube 1 was 8 mm. The heatingtemperature was 310° C. and the pulling speed was 2 mm/min. The outerdiameter of the preform 11 obtained was 6.5 mm, and, similarly to thepreform obtained in Example 1, neither devitrification of glass norgeneration of bubbles were observed. The adhesion between the core glassand the clad glass was excellent.

The same procedure as above was repeated, except that the preform 11obtained above was used as the core rod 2 and the glass composition ofthe cladding tube 3 was adjusted to Ge 2 atm %/As 37 atm %/S 61 atm %(the first composition). In this case, the outer diameter and the innerdiameter of the cladding tube 3 were 15.5 mm and 7.5 mm, respectively,the outer diameter of the core rod 2 was 6.5 mm, the aperture of thenozzle 5 of the quartz tube 1 was 8 mm. The heating temperature was 310°C. and the pulling speed was 2 mm/min. The preform 11 obtained had anouter diameter of 6.5 mm and, similarly to the preform obtained inExample 1, neither devitrification of glass and generation of bubbleswere observed. The adhesion between the core glass and the claddingclass was excellent.

The same procedure as above was repeated twice, except that the preform11 obtained above was used as the core rod 2. This procedure wasrepeated, except that the preform 11 thus obtained was used as the corerod 2 and the glass composition of the cladding tube 3 was adjusted toGe 1.5 atm %/As 38 atm %/S 60.5 atm % (the second composition). Thepreform 11 thus obtained had an outer diameter of 6.5 mm and, similarlyto the preform obtained in Example 1, neither devitrification of glassnor generation of bubbles were observed. The adhesion between the coreglass and the cladding glass was excellent.

As stated above, the above procedure was repeated five times in total toobtain a preform 11 as shown in FIG. 3 in which the outer diameter ofthe core glass 17 was 0.12 mm, the outer diameter of the cladding glass18 of the first composition consisting of four layers was 2.6 mm and theouter diameter of the cladding glass 19 of the second composition was6.5 mm.

(2) Glass Fiber-Production Example

The periphery of the preform 11 thus obtained was coated with aheat-shrinkable tube 12 composed of a heat-shrinkable fluoroplastic(Teflon FEP) and the lower periphery of the preform 11 was heated by aring heater 13 to 390° C. and then drawn into a single mode glass fiber14. The cross-sectional view of the single mode glass fiber 14 was asshown in FIG. 4. As is clear from FIG. 4, in this single mode glassfiber 14, the outer diameter of the core glass 17 was 2.3 μm, the outerdiameter of the cladding glass 18 of the first composition was 50 μm,the outer diameter of the clad glass 19 of the second composition was125 μm, the outer diameter of the fiber including the heat-shrunk resinlayer 20 was 135 μm. The transmission loss of the glass fiber 14 was0.08 dB/m at a wavelength of 2.4 μm.

Example 5

Using a production apparatus as shown in FIG. 5, a chalcogenide glassfiber having a core-cladding structure was produced.

In the apparatus for producing a chalcogenide glass fiber shown in FIG.5, a quartz crucible 107 having the largest diameter in the middle isprovided. Under the crucible 107, there are arranged an inert gas inlet111, a local heater 117, an atmosphere conditioning chamber 112, a fiberdiameter measuring instrument 113, a resin coater 114, a UV irradiationchamber 115 and print rollers 116. At the upper end of the crucible 107,a cover 103 having an inlet 109 for a pressurizing inert gas and asuction port 110 are placed and fixed by means of a resin tube 104.

A resin tube 106 is connected to the suction port 110 and the lower endof the resin tube 106 is connected to the upper end of a cladding tube102 inserted into the crucible 107 by means of a resin tube 105 to holdthe cladding tube 102. The crucible 107 has at its bottom a nozzle 108having an aperture smaller than the outer diameter of the cladding tube102 but larger than the outer diameter of the core rod 101.

A core glass having a composition of As 40 atm %/S 60 atm % was polishedto form a core rod 101 having an outer diameter of 8 mm, and a claddingglass having a composition of As 34 atm%/S 62 atm %/Ge 4 atm % waspolished to form a cladding tube 102 having an outer diameter of 10 mmand an inner diameter of 8.5 mm. The core rod 101 was inserted into thecladding tube 102. The resulting assembly consisting of the core rod 101and the cladding tube 102 was set in the crucible 107 having a nozzle108 at its bottom, and the crucible 107 was purged with an argon gas.

Subsequently, the vicinity of the lower end of the crucible 107 waslocally heated by a local heater 117 to such a temperature that theviscosities of the glasses of the cladding tube 102 and the core rod 101present therein became 10⁶ poises to fusion-bond the cladding tube 102to the core rod 101 in the vicinity of the lower end of the crucible 107and simultaneously uniformly melt the cladding tube 102 at the peripheryof the nozzle 108 at the lower end of the crucible 107.

A pressure of 1.47×10⁵ Pa was applied to the periphery of the claddingtube 102 and simultaneously the pressure of the space between thecladding tube 102 and the core rod 101 was reduced to 1.33 Pa tocompletely unite the cladding tube 102 with the core rod 101, afterwhich the united glass was continuously drawn into a glass fiber havinga core diameter of 200 μm and a clad diameter of 250 μm.

Just after the spinning, the fiber was coated with a UV curable resin bymeans of a resin coater 114 and then the UV curable resin was cured in aUV irradiation chamber 115, after which the fiber was wound around adrum (not shown in the figures). The fiber thus obtained was subjectedto the measurements to find that the transmission loss at a wavelengthof 2.4 μm was 0.1 dB/m, the tensile strength was as high as 15 kg/mm²,and the number of aperture (NA) was 0.5.

The chalcogenide glass fiber thus obtained had such a structure that thecore glass 120 composed of two elements of As (arsenic) and S (sulfur)was covered with a cladding glass 121 composed of three elements of As(arsenic), S (sulfur) and Ge (germanium) as shown in FIG. 6.

Example 6

A core glass having a composition of As 40 atm %/S 60 atm % was polishedto form a core rod having an outer diameter of 2 mm, and a firstcladding glass having a composition of As 34 atm %/S 62 atm %/Ge 4 atm %was polished to form a first cladding tube having an outer diameter of50 mm and an inner diameter of 3 mm. Moreover, a second cladding glasshaving a composition of As 37 atm %/S 62 atm %/Ge 1 atm % was polishedto form a second cladding tube having an outer diameter of 110 mm and aninner diameter of 55 mm. The first cladding tube was inserted into thesecond cladding tube and the core rod was inserted into the firstcladding tube, after which the resulting assembly was set in thecrucible 107 in the same apparatus as in Example 5. In the same manneras in Example 5, the assembly was heated and continuously drawn into asingle mode fiber having a core diameter of 2.4 μm and a clad diameterof 125 μm.

The resulting fiber was coated with a resin in the same manner as inExample 5 just after the drawing and wound around a drum. As a result,the refractive index of the second cladding glass became higher thanthat of the first cladding glass but lower than that of the core glass.The fiber was subjected to measurements to find that the transmissionloss at a wavelength of 2.4 μm was as low as 0.1 dB/m, the tensilestrength was as high as 14 kg/mm², and the number of aperture (NA) was0.5. Substantially no disturbance light having passed through thecladding glass was observed.

The chalcogenide glass fiber thus obtained had such a structure that afiber composed of a core glass 120 whose periphery was covered with thecladding glass 121 was further covered with a second cladding glass 122composed of three elements of As (arsenic), S (sulfur) and Ge(germanium) as shown in FIG. 7.

Examples 7 to 15 and Reference Example

In the same manner as in Example 5 or 6, glasses having the compositionsshown in Table 1 (Examples 7 to 15 and Reference Example) were drawninto fibers and the results of measurements of the fibers are shown inTable 2. The unit in Table 1 is atm % and the loss in Table 2 is atransmission loss (unit: dB/m) at a wavelength of 2.4 μm. The unit ofstrength is kg/mm². The Reference Example in the last column of theTables is an example in which both the core glass and the clad glasscontained Ge.

                  TABLE 1    ______________________________________    Example           Core glass   lst Clad glass                                    2nd clad glass    No.    As    S     Ge  Se   As  S    Ge   As    S   Ge    ______________________________________     5     40    60             34  62   4    --    --  --     6     40    60             34  62   4    37    62  1     7     26    74             19  75   6    20    76  4     8     54    46             48  48   4    51    47  2     9     48    52             46  52   2    47    52  1    10     42    58             39  57   4    --    --  --    11     35    65             34  65   1    34.5  65  0.5    12     38    62             36  60   4    --    --  --    13     45    55             40  53   7    --    --  --    14     31    69             30  68   2    30    69  1    15     40    60             36  61   3    --    --  --    Ref.   20    58    20  2    20  60   20   --    --  --    Ex.    ______________________________________

                  TABLE 2    ______________________________________    Example                     Disturbance    No.       Loss   Strength   light   NA    ______________________________________     5        0.1    15         Yes     0.5     6        0.1    14         No      0.5     7        0.1    14         No      0.6     8        0.2    15         No      0.5     9        0.1    14         No      0.3    10        0.1    14         Yes     0.5    11        0.1    15         No      0.2    12        0.1    16         Yes     0.5    13        0.1    14         Yes     0.6    14        0.1    14         No      0.3    15        0.1    14         Yes     0.4    Ref.      0.3    11         Yes     0.4    Ex.    ______________________________________

From the above results, it can be seen that as is clear from comparisonwith the Reference Example, this invention using a core glass free fromGe and a clad glass containing Ge is significantly lower in transmissionloss and higher in tensile strength. In particular, when the core glasscomposition is such that the proportion of As is in the range of 25 to55 atm % and the proportion of S is in the range of 45 to 75 atm % andwhen the cladding glass composition is in such a range that theproportion of As is in the range of 18 to 54 atm %, the proportion of Sis in the range of 46 to 80 atm % and the proportion of Ge is in therange of 0.5 to 7 atm %, such effects are obtained that the transmissionloss is reduced and the mechanical strength is enhanced.

In Examples 7, 8, 9, 11 and 14 in which the fiber is covered with thesecond cladding glass, the refractive index of the second cladding glassbecomes higher than that of the first cladding glass and lower than thatof the core glass. In particular, when the second cladding glasscomposition is such that the proportion of As is in the range of 18 to54 atm %, the proportion of S is in the range of 46 to 80 atm % and theproportion of Ge is in the range of 0.5 to 7 atm %, it has beenconfirmed that the disturbance light is effectively prevented.

What is claimed is:
 1. A process for producing a preform for achalcogenide glass fiber which comprises the steps of:(a) inserting acladding chalcogenide glass tube, having contained therein achalcogenide glass rod for core, into a quartz tube having at its bottoma nozzle having an aperture smaller than an outer diameter of thecladding tube, (b) locally heating the bottom of the quartz tube tosoften a chalcogenide glass to a temperature 40-150° C. below thedrawing temperature, and (c) hanging a weight from a tip of the preformand pulling out the cladding tube having contained therein and adheredthereto the glass rod core.
 2. The process for producing a preform for achalcogenide glass fiber according to claim 1, wherein the pressure inthe space between the glass rod for core and the cladding tube isadjusted relatively lower than the pressure in the space between thecladding tube and the quartz tube.
 3. A process for producing a preformfor a chalcogenide glass fiber which comprises(a) inserting a claddingchalcogenide glass tube, having contained therein a chalcogenide glassrod for core, into a quartz tube having at its bottom a nozzle having anaperture smaller than an outer diameter of the cladding tube, (b)locally heating the bottom of the quartz tube to soften a chalcogenideglass to a temperature 40-150° C. below the drawing temperature, and (c)hanging a weight from a tip of the preform and pulling out the claddingtube having contained therein and adhered thereto the glass rod core,(d) inserting into a second cladding tube the preform of step (c) as aglass rod, (e) repeating the process of steps (a), (b) and (c) at leastone time to obtain a preform devoid of gaps between the cladding layersand between the innermost cladding layer and the glass rod for a singlemode fiber having a very small core diameter as compared with a claddingdiameter.
 4. A process for producing a chalcogenide glass fiber whichcomprises polymer-coating a preform for a chalcogenide glass fiberobtained by the process according to claim 1 with a polymer-coatingmaterial and then heating and drawing the coated preform into a fiber.5. The process for producing a chalcogenide glass fiber according toclaim 4, wherein all of the preform is coated with a polymer-coatingmaterial and then heated and drawn into a glass fiber having a triplelayer structure of core glass, cladding glass and polymer-coatingmaterial.
 6. The process for producing a chalcogenide glass fiberaccording to claim 4, wherein the preform except for its tip area iscoated with a polymer-coating material and then the preform was heatedand drawn into a coat-free fiber having a double layer structure of coreglass and cladding glass.
 7. A process for producing a chalcogenideglass fiber provided with a protective coating layer, which comprisespassing through a coating cup with a nozzle at its bottom, achalcogenide glass fiber obtained by drawing a preform for achalcogenide glass fiber obtained by the process according to claim 1.8. A process for producing a chalcogenide glass fiber which comprisesthe steps of:(a) inserting a cladding chalcogenide glass tube, havingcontained therein a chalcogenide glass rod for core, into a quartz tubehaving at its bottom a nozzle having an aperture smaller than an outerdiameter of the cladding tube, (b) locally heating the bottom of thequartz tube at a temperature which is lower than the drawing temperatureand at which the cladding glass united with the core glass can be pulledout from the nozzle of the quartz tube without causing crystallizationin the core glass and at the core glass-cladding glass interface,pulling out the cladding tube having contained therein and adheredthereto the glass rod core to produce a preform, polymer coating thepreform of step (c), except for its tip, with a polymer coatingmaterial, and thereafter heating and drawing the preform into acoat-free glass fiber having a double layer structure of core glass andcladding glass.
 9. A process for producing a chalcogenide glass fiberwhich comprises:(a) inserting a cladding chalcogenide glass tube, havingcontained therein a chalcogenide glass rod for core, into a quartz tubehaving at its bottom a nozzle having an aperture smaller than the outerdiameter of the cladding tube, (b) locally heating the bottom of thequartz tube at a temperature which is lower than the drawing temperatureand at which the cladding glass united with the core glass can be pulledout from the nozzle of the quartz tube without causing crystallizationin the core glass and at the core glass-cladding glass interface, (c)pulling out the cladding tube having contained therein and adheredthereto the glass rod core, (d) inserting into a second cladding tubethe preform of step (c) as a glass rod, (e) repeating the process ofsteps (a), (b) and (c) at least one time to obtain a preform devoid ofgaps between the cladding layers and between the innermost claddinglayer and the glass rod for a single mode fiber having a very small corediameter as compared with a cladding diameter, (f) polymer coating thepreform of step (e), except for its tip, with a polymer coatingmaterial, and thereafter (g) heating and drawing the preform into acoat-free glass fiber having a triple layer structure of core glass,cladding glass and polymer coating material.